The present invention relates to implantable medical devices, and more particularly to implantable micro stimulators or sensors, hereafter referred to as microstimulators or microsensors. Such devices have electrodes attached to muscle or nerve fibers, through which the devices electrically stimulate the muscle or nerve fibers, or sense one or more physiological states present in the muscle or nerve fibers. More particularly, the invention relates to an improved antenna for such implantable microdevices, for both receiving signals from an external device, and transmitting signals to an external device.
Neurological disorders are often caused by neural impulses failing to reach their natural destination in otherwise functional body systems. Local nerves and muscles may function, but, for various reasons, injury, stroke, or other cause, the stimulating signals do not reach their natural destination.
For example, paraplegics and quadriplegics have intact muscles and only lack the complete brain-to-muscle nerve link which conducts the signal to the muscles.
Prosthetic devices have been used for some time to provide electrical stimulation to excite muscle, nerve or other cells to provide relief from paralysis, and various other physical disorders have been identified which may be treated by electrical stimulation devices. Some of these devices have been large bulky systems providing electrical pulses through conductors extending through the skin. Disadvantageously, complications, including the possibility of infection, arise in the use of stimulators which have conductors extending through the skin.
Other smaller stimulators have been developed that are fully implantable and are controlled through high-frequency, modulated RF, telemetry signals. Such systems designed to stimulate nerves or muscles to provide motion are know as Functional Electrical Stimulation (FES) systems. An FES system using telemetry signals is set forth in U.S. Pat. No. 4,524,774, issued Jun. 25, 1985 for xe2x80x9cApparatus and Method for the Stimulation of a Human Muscle.xe2x80x9d The ""774 patent teaches a source of electrical energy, modulated in accordance with desired control information, to selectively power and control numerous, small stimulators, disposed at various locations within the body. Thus, for example, a desired progressive muscular motion may be achieved through the successive or simultaneous stimulation of numerous stimulators, directed by a single source of information and energy outside the body.
Many difficulties arise in designing implanted stimulators which are small in size, and in passing sufficient energy and control information to the stimulators to satisfactorily operate them without direct connection. A design of a small functionally suitable stimulator, a microstimulator, is taught is U.S. Pat. No. 5,324,316 issued Jun. 28, 1994 for xe2x80x9cImplantable Microstimulator.xe2x80x9d The ""316 patent teaches all the elements required for successful construction and operation of a microstimulator. The microstimulator is capable of receiving and storing sufficient energy to provide the desired stimulating pulses, and also is able to respond to received control information defining pulse duration, current amplitude and shape. The microstimulator of the ""316 patent can also be easily implanted, such as by expulsion through a hypodermic needle. The ""316 patent is incorporated herein by reference.
Known microstimulators utilize a telemetry receiver based on modulating an inductive power signal provided to the microstimulator. Similarly, signals are back transmitted from the microstimulator using the same circuits. By using components already present in the microstimulator, these telemetry systems do not require substantial additional circuitry. However, such inductive telemetry methods are limited by the resonant frequencies of the existing coil, which are typically below 2 MHz. While this approach has proven adequate for many applications, there are potential problems with interfering signals. Further, much higher frequencies, 402 MHz to 405 MHz, have been designated by the Federal Communications Commission (FCC) for use with medical devices.
Telemetry methods utilizing monopole and dipole antennas are known for use in the FCC designated frequency range, however, such antennas are, primarily, electrical field devices. Electrical field devices suffer from high tissue detuning (i.e., the surrounding tissue interacts with the electrical nature of circuit components to the extent that some effectiveness of tuning is lost) and may not provide the best performance for implantable devices. Other telemetry systems utilizing a loop antenna inside the microdevice are also known in the art. Loop antennas have the advantage of being magnetic field devices, and are therefore less susceptible to tissue detuning. However, placing the loop antenna inside the case of a microdevice exhausts scarce space within the microdevices.
What is needed is a telemetry system, suitable for operation in the 402 MHz to 405 MHz frequency range, that does not suffer from high tissue detuning loss, and that does not take up substantial space within the implantable microdevice.
The present invention addresses the above and other needs by providing a loop antenna formed on the case of an implantable microdevice. The improved antenna receives data transmitted from an external device, and transmits data to an external device. Such a loop antenna may be formed from two cylindrical sections separated by an insulating material on the case of the microdevice, or by separating a metal cylinder into two parallel semi-cylinders separated by an insulating material. A tuning circuit comprising capacitors and/or varactors is used to obtain resonance in the loop antenna, thus creating a sufficiently large effective antenna aperture. Advantageously, such a loop antenna is suitable for operation in the 402 MHz to 405 MHz frequency range, is a magnetic field device and therefore not susceptible to high absorption losses, and does not require space in the interior of the microdevice.
In accordance with one aspect of the invention, a loop antenna is formed on the case of an implantable microdevice. By forming the antenna on the case, space inside the microdevice is available for circuit components. In one embodiment of the invention, the existing electrodes, on the case of a microstimulator, are combined with a reactive circuit to create a loop antenna.
It is a feature of the invention to provide an implantable medical device having a loop antenna, which loop antenna is advantageously a magnetic field device. Magnetic field devices are less prone to degradation from tissue absorption than are electrical field devices, such as dipole and monopole antennas. Accordingly, once implanted, a magnetic field device is more stable and predictable than an electrical field device.
In accordance with another aspect of the invention, a loop antenna provided in an implantable medical device may be tuned with an array of capacitors and/or varactors. Because of the small physical size of the antenna, the antenna is not an effective radiator at the targeted operating frequencies without tuning. Accordingly, the capacitance provided by an array of capacitors and/or varactors is adjusted to be equal to the inductive reactance of the loop, resulting in a high Q circuit and a larger effective antenna size.
In accordance with yet another aspect of the invention, a telemetry system using a loop antenna provides non-inductive telemetry capability. Inductive telemetry requires that the transmitter and receiver be in very close proximity for effective operation. The telemetry system provided by the loop antenna does not include such limitations.